﻿CRYSTALLINE 
  ROCKS 
  OP 
  THE 
  MALVERN 
  HILLS. 
  4S3 
  

  

  (No. 
  1 
  of 
  my 
  former 
  paper, 
  loc. 
  cit. 
  p. 
  526), 
  the 
  most 
  abundant 
  variety 
  

   of 
  diorite 
  in 
  the 
  Malvern 
  Hills. 
  It 
  is 
  penetrated 
  irregularly 
  by 
  a 
  few 
  

   granite-veins. 
  Towards 
  the 
  left, 
  as 
  we 
  face 
  the 
  section, 
  veins 
  of 
  

   granite 
  come 
  in 
  more 
  numerously. 
  Signs 
  of 
  pressure 
  also 
  appear, 
  

   and 
  increase 
  towards 
  the 
  zone 
  in 
  which 
  the 
  mixed 
  diorite 
  and 
  

   granite 
  are 
  rolled 
  out 
  into 
  a 
  banded 
  micaceous 
  gneiss. 
  The 
  rock 
  

   above 
  the 
  level 
  reached 
  by 
  most 
  of 
  the 
  veins 
  in 
  the 
  sheared 
  part 
  

   is 
  schistose, 
  but 
  much 
  decomposed 
  *. 
  Further 
  back 
  towards 
  the 
  

   unsheared 
  end, 
  where 
  the 
  veins 
  are 
  fewer, 
  sound 
  diorite 
  appears 
  

   within 
  a 
  yard 
  or 
  so 
  of 
  the 
  upward 
  termination 
  of 
  the 
  veins. 
  The 
  

   unsound 
  rock, 
  which 
  here 
  (and 
  frequently 
  elsewhere) 
  forms 
  a 
  

   sort 
  of 
  aureole 
  round 
  a 
  plexus 
  of 
  veins, 
  is 
  largely 
  micaceous 
  and 
  

   chloritic. 
  

  

  The 
  following 
  series 
  of 
  slides 
  illustrates 
  the 
  transition 
  from 
  diorite 
  

   to 
  mica-gneiss. 
  By 
  the 
  " 
  edge 
  " 
  or 
  " 
  margin 
  " 
  of 
  a 
  shear-zone 
  or 
  

   sheared 
  complex, 
  I 
  merely 
  mean 
  the 
  boundary-line 
  between 
  the 
  

   undoubted 
  gneiss 
  of 
  the 
  zone 
  and 
  the 
  more 
  or 
  less 
  sheared 
  rock 
  

   which 
  graduates 
  up 
  to 
  it. 
  

  

  Four 
  specimens 
  (273-276) 
  were 
  taken 
  near 
  together 
  from 
  the 
  

   diorite 
  3 
  feet 
  from 
  the 
  zone. 
  They 
  consist 
  of 
  about 
  half 
  hornblende 
  

   and 
  half 
  felspar. 
  The 
  crystals 
  of 
  the 
  former 
  rarely 
  present 
  perfect 
  

   forms, 
  being 
  usually 
  rather 
  jagged 
  or 
  corroded 
  at 
  the 
  margin. 
  Some 
  

   have 
  undergone 
  mineral 
  change, 
  having 
  liberated 
  iron-oxide 
  as 
  an 
  

   opaque 
  stain, 
  or 
  being 
  partly 
  converted 
  into 
  chlorite, 
  especially 
  along 
  

   cleavage-planes. 
  Plagioclase 
  can 
  be 
  here 
  and 
  there 
  detected, 
  but 
  

   much 
  of 
  the 
  felspar 
  displays 
  no 
  twinning. 
  In 
  some 
  of 
  the 
  crystals 
  

   are 
  many 
  microliths 
  of 
  a 
  clear 
  transparent 
  mineral, 
  probably 
  white 
  

   mica, 
  and 
  larger 
  patches 
  of 
  white 
  mica. 
  There 
  is 
  some 
  cubic 
  iron-ore 
  

   and 
  a 
  little 
  sphene, 
  the 
  latter 
  either 
  free 
  or 
  incrusting 
  ilmenite. 
  Nos. 
  

   274 
  and 
  276 
  are 
  cut 
  respectively 
  from 
  the 
  same 
  hand-specimens 
  as 
  273 
  

   and 
  275, 
  at, 
  or 
  near, 
  contact 
  with 
  small 
  granite-veins. 
  They 
  show 
  a 
  

   tendency 
  towards 
  parallelism 
  and 
  aggregation 
  of 
  the 
  constituents. 
  

   This 
  is 
  especially 
  seen 
  in 
  the 
  hornblendes, 
  which, 
  here 
  and 
  there 
  occur 
  

   in 
  folia 
  of 
  from 
  half 
  a 
  dozen 
  to 
  a 
  dozen 
  crystals 
  variously 
  orientated. 
  

   Some 
  of 
  the 
  crystals 
  are 
  also 
  much 
  larger 
  than 
  in 
  273 
  and 
  275. 
  In 
  

   respect 
  to 
  parallelism, 
  aggregation, 
  and 
  enlargement, 
  the 
  rock 
  close 
  

   to 
  the 
  veins 
  forms 
  a 
  connecting-link 
  between 
  the 
  ordinary 
  diorite 
  

   and 
  some 
  of 
  the 
  gneissic 
  rocks 
  which 
  follow 
  in 
  the 
  series. 
  

  

  Nos. 
  311 
  and 
  312, 
  taken 
  18 
  inches 
  from 
  the 
  shear-zone, 
  show 
  

   characters 
  similar 
  to 
  those 
  of 
  274 
  and 
  276, 
  with 
  the 
  following 
  

   differences 
  : 
  — 
  The 
  slides 
  are 
  altogether 
  much 
  clearer. 
  The 
  hornblende 
  

   lies 
  in 
  folia 
  of 
  irregular 
  thickness, 
  and 
  the 
  crystals 
  are 
  sometimes 
  

   drawn 
  out 
  in 
  the 
  direction 
  of 
  schistosity. 
  They 
  frequently 
  present 
  

   curvilinear 
  outlines 
  in 
  a 
  very 
  marked 
  degree, 
  and 
  are 
  perforated 
  by 
  

   holes 
  usually 
  occupied 
  by 
  a 
  water-clear 
  mineral. 
  I 
  suppose 
  these 
  

   peculiarities 
  are 
  the 
  result 
  of 
  corrosion 
  ; 
  but 
  if 
  they 
  are 
  produced 
  

   during 
  recrystallization 
  under 
  new 
  conditions, 
  the 
  distinction 
  is 
  not 
  

   very 
  material 
  ; 
  they 
  do 
  not 
  characterize 
  the 
  unmodified 
  diorite, 
  and 
  

  

  * 
  This 
  unsoundness, 
  occurring, 
  as 
  it 
  usually 
  does 
  at 
  Malvern, 
  at 
  the 
  most 
  

   critical 
  points, 
  renders 
  it 
  extremely 
  difficult 
  to 
  obtain 
  satisfactory 
  results. 
  

  

  2m 
  2 
  

  

  